A significant contributor to the morbidity, mortality, health status impact and cost of care of chronic obstructive pulmonary disease (COPD) are intermittent exacerbations, Current optimal treatment of COPD prevents only 40% of exacerbations and as many as 25% of patients with an exacerbation either display incomplete resolution or relapse within 1 month. Most exacerbations are caused by bacterial and viral airway infection. Therefore, to improve our management of COPD exacerbations, it is important to elucidate precisely the extent of bacterial and viral infection in COPD and its relation to airway inflammation, as proposed in this research application. Central to the proposal is continuation of a longitudinal prospective cohort study in 50 patients with COPD that has been supported by this Merit Review since 1994. Participants are seen every 4 weeks in the Study Clinic and at exacerbation. At each visit the patient is evaluated clinically and sputum and serum sample are obtained. Specific Aims and planned experiments are as follows. Molecular techniques will be used to characterize bacterial infection in COPD and results will be compared with culture. DNA will be extracted from sputum samples obtained on a monthly basis in the COPD study clinic and used as template in real time PCR (RT-PCR) to make a quantitative determination of the presence of the 4 major pathogens, H. influenzae, S. pneumoniae, M. catarrhalis and P. aeruginosa. In order to determine the significance of pathogen detection, airway inflammation will be quantified and immune response to bacterial pathogens following exacerbations will be examined. Novel observations will include a comparison of molecular diagnostic techniques to COPD with cultures, and an understanding of the implication of detecting a pathogen only by molecular techniques. Symptomatic resolution of exacerbations will be assessed objectively with a validated patient reported outcome measure, the EXACT-PRO. Whether this clinical resolution is determined by eradication of the offending pathogen (bacteria or virus) and by resolution of airway and systemic inflammation will be assessed. When an exacerbation is diagnosed in a clinic visit, a total of 5 sputum samples and 3 serum samples will be obtained within 28 days, as well as daily electronic recording of the EXACT-PRO. Sputum specimens will be subjected to bacterial and viral detection. A comprehensive assessment of airway inflammation in sputum will also be made. Serum c-reactive protein and fibrinogen will be used to measure systemic inflammation. Whether EXACT-PRO resolution of exacerbation is related to microbial eradication and inflammatory resolution will be analyzed. A careful study of infection, inflammation and clinical symptoms will provide new information as to how these are linked in exacerbations. It would also inform whether better antimicrobial or anti-inflammatory therapies are needed to improve outcomes. The relationship of acquisition of new strains of Enterobacteriaceae and Staphylococcus aureus with exacerbations will be systematically examined. Systemic immune response to the isolated strains and airway inflammation with strain acquisition will be systematically assessed. The Enterobacteriaceae strains will be typed by REP-PCR. The S. aureus strains will be characterized by spa typing. Following strain characterization as new or pre-existing, the relationship between new strain acquisition and exacerbation will be determined. Inflammatory and immune response in exacerbations associated with new strains of these pathogens will be studied. This will provide a rigorous evaluation of the role of Enterobacteriaceae gram negative bacilli and S. aureus in exacerbations and inform us of the need for treatment of these pathogens in exacerbations.